The viscoelasticity of the retina can reflect its function and state. Accurate assessment of retinal viscoelasticity can assist in early diagnosis of retinal diseases. With high-resolution and non-contact features, optical coherence elastography (OCE) has been used to evaluate the retinal elasticity based on the elastic wave velocity measurement. Nonetheless, the retinal viscosity cannot be assessed. In this study, a shear wave dispersion OCE method was proposed to measure the retinal viscosity and elasticity. After acoustic radiation force (ARF) induces a shear wave, optical coherence tomography (OCT) visualizes shear wave propagation in the retina. The wave velocities at different frequencies are analyzed, and the viscoelasticity is quantified based on the dispersion analysis. The accuracy of the method was verified on phantoms with different glycerol concentrations. The viscosity of the phantom is related to the concentration of glycerin, and its elasticity is adjusted by the agar concentration. The OCE results closely matched the elasticity measured by a mechanical testing system. Furthermore, the retinal shear wave velocity dispersion on the ex-vivo porcine eye was analyzed to determine its viscoelasticity. Our results demonstrate that ARF-OCE can quantitatively evaluate the viscoelasticity of the retina. The shear wave dispersion OCE method has great potential for diagnosing retinal diseases.
Optical coherence tomography (OCT) is a non-invasive, label-free imaging modality that generates high-resolution threedimensional images. Based on OCT imaging, optical coherence tomography angiography (OCTA) and optical coherence elastography (OCE) can visualize vascular networks and measure the elastic properties of biological tissues. In previous studies, OCTA and OCE were performed separately, providing either vascular network information or elasticity properties of the tissue. We have developed a simultaneous angiography and elastography method using a simplified sample arm structure. After mechanical pressure is loaded on the sample by a glass plate, the deformation is analyzed by the OCT phase changes, and the elasticity is assessed. Meanwhile, the vascular network is visualized by intensity-based Doppler variance analysis. A transparent flexible reference layer is placed between the glass plate and the tissue, which closely contacts the tissue. Better elasticity measurements can be achieved without affecting vascular imaging. The simultaneous elastography and angiography method was demonstrated by the phantom experiments and rat skin measurements. The results show that the information on the microvascular networks and mechanical properties can be obtained at the same imaging location. The method can provide more comprehensive information on biological tissue for disease diagnosis and treatment monitoring.
Optical coherence tomography angiography (OCTA) is a label-free, high-resolution imaging technique for detecting blood flow based on optical coherence tomography (OCT) and time-series signal analysis. In OCTA, the time-series signals at the same position are captured, and the changes in the signals are analyzed to detect the blood flow. In this study, we evaluated different scan protocols for the OCTA regarding image quality and sampling time, including the dense A-scan, dense B-scan, and multiple B-scan protocols. In the dense A-scan or the dense B-scan protocols, the beam continues scanning with a slight change between adjacent positions. Whereas, the scan beam will pause at each slow scan position to repeatedly capture the B-scans in the multiple B-scan protocol. After the time-series signals were captured using different scan protocols and analyzed using an OCTA algorithm, the vasculature of the rat tissue was visualized. The image quality was analyzed to assess the efficiency of the scan protocols. The quantitative evaluation of the scan protocols allows for optimizing the sampling schemes in the OCTA imaging of biological tissues.
The biomechanical measurement of the crystalline lens can provide valuable information to assess the development of lens-related diseases, such as presbyopia and cataracts. Optical coherence elastography (OCE) has been used to measure the elasticity of the lens surface based on elastic wave imaging. However, measuring the elasticity of the lens interior poses a challenge because optical imaging cannot easily visualize elastic waves in the transparent lens. In this study, we develop an acoustic radiation force optical coherence elastography (ARF-OCE) method to detect the propagation of elastic waves on the surface of the lens and inside the lens for the elasticity measurement. The ultrasonic radiation force excites the lens from the side of the eye, subsequently inducing an elastic wave on the lens surface or inside the lens. Optical coherence tomography (OCT) images the crystalline lens from the front of the eye with the optical beam perpendicular to the acoustic beam. When the ARF is focused on the surface of the lens, the wave propagation on the lens surface is visualized by the OCT, and the elasticity of the lens surface can be quantified. When the ARF is focused inside the lens at different depths, the time the elastic wave reaches the lens surface will change. Therefore, the velocity of the elastic wave propagation inside the lens is calculated by the ratio of the depth change to the time difference, and the elasticity of the lens interior can be quantified. The elasticity of the surface and the interior of the ex-vivo porcine lens was measured using the ARF-OCE method. The elasticity measurement of the crystalline lens provides a quantitative assessment of its biomechanical properties and has the potential for the accurate diagnosis and treatment of lens-related diseases.
Optical coherence elastography (OCE) is an imaging technique for measuring elastic properties based on optical coherence tomography (OCT). Benefitting from the high-resolution imaging and sensitive motion detection of OCT, OCE has been used to assess the elasticity distribution of ocular tissue, skin tissue, etc. The biological tissues consist of multiple layers with various biomechanical properties and, thus, show directionally dependent elasticity. In previous OCE measurements, the tissues were regarded as isotropic materials, and the elasticity was measured in one direction, which may cause the bias of elasticity assessment. In this study, we developed the OCE method for directionally dependent elasticity assessment. Acoustic radiation force (ARF) generated by a ring ultrasonic transducer was applied to induce vibration in an axial direction and an elastic wave propagating in the lateral direction. The OCT beam, parallel to the ARF, scanned the sample through the central hole of the ring ultrasonic transducer. Then, phase-resolved OCT analysis was used to detect the vibration and visualize wave propagation. The vibration amplitude depends on the axial elasticity, and the lateral elasticity determines the elastic wave velocity. Therefore, the measurements of the vibration amplitude and the wave velocity using ARF-OCE can assess the elasticity of the sample in orthogonal directions. The results from the phantom measurements show that the OCE method can reliably evaluate the directionally dependent elasticity for the anisotropic samples.
Optical coherence tomography (OCT) is a non-invasive, label-free imaging technique with high resolution. Due to the relatively low scan rate of OCT and involuntary bulk motion of tissues, the OCT image will be distorted by the motion artifacts. The motion artifacts can be reduced by hardware and software methods. In hardware methods, additional hardware is used to track the motion of the object, and extra scans may be required during data acquisition. The software methods can simplify the device and the data acquisition. However, the motion correction based on the cross-correlation analysis is time-consuming. In this study, we proposed a fast motion correction method for OCT images based on image feature matching. First, the motion-related mismatch in the slow scan direction was compensated by the image feature matching between the adjacent B-scans based on the oriented FAST and rotated BRIEF (ORB) approach. Then the axial motion in A-scans was corrected by the boundary detection of the tissue structure and the non-rigid transformation between the corresponding A-lines in the adjacent B-scans. The fast motion correction method was validated by the OCT imaging of a rat ear. The results show that the method can effectively correct motion artifacts of OCT images with a fast processing speed.
Optical coherence tomography (OCT) enables high-resolution, label-free two-dimensional cross-sectional and three-dimensional volumetric imaging of biological tissues. Combining OCT imaging with external force excitation, optical coherence elastography (OCE) provides noninvasive elasticity quantification of samples for the pathological analysis of tissues and early diagnosis of diseases. However, the OCE system with a fixed OCT sample arm cannot be used for elasticity measurements of tissues located in a narrow space, such as an oral cavity and an ear canal, because the OCT beam and external force cannot easily reach the tissues. In this study, we developed a handheld OCE method for the elasticity measurements based on elastic wave imaging. The handheld probe integrated an air pulse excitation unit and a microelectromechanical system-based scan imaging unit. A short air pulse induced the elastic wave in a sample. Then the OCT data was captured by an M-B scan protocol, and the tissue vibration was analyzed by Doppler phase shifts. After elastic wave visualization, the elastic wave velocity was calculated for the elasticity quantification of the sample. The results show that the handheld OCE method can induce and image the elastic wave and, thus, quantify the elastic modulus with high flexibility for the tissue in a narrow, deep space.
The elastic properties can be an indicator of pathological changes of biological tissue. Acoustic radiation force optical coherence elastography (ARF-OCE) allows remote, non-invasive assessment of the elastic properties of tissue. In this study, we proposed an acousto-optic coupling ARF-OCE method for the elasticity measurements. The acousto-optic coupling unit employs a rectangular prism with a close refractive index but significantly different acoustic impedance compared to water. Therefore, the surface of the rectangular prism immersed in water can reflect the ultrasound beam while transmitting the optical coherence tomography (OCT) detection beam. We demonstrated the acousto-optic coupling ARFOCE method using agar phantoms. The results show that the ARF-OCE method can induce elastic vibrations in the direction parallel to the OCT beam, resulting in higher detection sensitivity and a larger scanning range.
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